When you're stretching in preparation for a workout, different people will give you varying opinions about whether it is better for your body to hold a steady stretch for a longer period of time or apply a deeper stretch for a shorter period.

"Which is going to be more effective? Which is better? Which will give you the least amount of damage?" asks Roderic Lakes, a professor of engineering mechanics and biomedical engineering.

With colleague Ray Vanderby Jr., an associate professor of surgery, engineering physics, biomedical engineering and mechanical engineering, Lakes is studying the viscoelasticity of ligaments. "Viscoelasticity means that it doesn't spring back right away when you remove a force from it, and if you keep a steady force, it will continue to deform," explains Lakes.

Initially, ligaments stretch easily, but as they stretch they become stiffer and there's a limit to how much stretching they can tolerate, he says. The pair's research group hopes to couple studies of creep (continued stretching during a steady force) with studies of relaxation (a decrease in resistance under a steady stretch deformation) at different load and strain levels.

Scientists have conducted experiments for each of these areas, but have not studied how they are interrelated. Rather, they viewed the time of loading and how much load is applied as separate factors, says Lakes. "They assumed that you can separate those out and write them as multiplied form," he says. "And it turns out, you can't. They're woven together."

An additional benefit from their studies of ligament stretching is that the research also will identify points at which ligaments are damaged. "How much it takes is going to depend on how fast the force is applied," says Lakes. "That is, the threshold for tearing tissue during a slow activity like weightlifting may be different from the threshold for tearing tissue during a sudden bump — say, during a ski run. That may only last a fraction of a second."

With their research, a three-year project funded by the National Science Foundation, the group is forging new experimental and theoretical ground, as well as developing principles they can expand beyond biological materials. "We're hoping that what we're doing on the theoretical side will have relevance to other materials besides ligaments," says Lakes. "For example, the metal in your lawnmower engine or outboard motor engine is hot enough that it'll also stretch out like a tendon. If it stretches out too much, then your engine may fail even before anything's worn out just because things have gotten loose."